Separation of discolorants from clay

ABSTRACT

A process for beneficiating a dispersed kaolin clay mass by removing from a dispersed clay mass the contaminant which causes the formation of a thin, dull layer of yellow or brownish-yellow material, commonly referred to as &#39;&#39;&#39;&#39;scum,&#39;&#39;&#39;&#39; on surfaces of dried material containing the dispersed clay. The process comprises raising the pH of an aqueous dispersion of the clay to at least 12 and at that pH separating the insoluble clay from the solubilized discoloring contaminant.

I United States Patent [151 3,659,708 Morris et al. 1 May 2, 1972 [s41 SEPARATION OF DISCOLORANTS 3,442,677 5/l969 Balentine ..22/110 x FROM CLAY 2,251,256 7/1941 Feldenheimer. .....l06/288 x 3,414,422 l2 i968 L ll' ..23 110 P [72] inventors: Horton H. Morris; James P. Ollvler, both anmce x o Ma o Sy y Ross. T y. N FOREIGN PATENTS 0R APPLICATIONS 1 g Freeport p ur C mpa y. N York, 886,653 l/l962 Great Britain ..23/110 l,l04,281 2/1968 Great Britain ....106/72 2 Filed: Nov. 4 19 1,420,612 1 H1965 France ..209/10 PP 3,335 Primary Examiner-Frank W. Lutter Assistant E.\'aminerRobert Helper 52 us. Cl. ..209/s,23/1 10p Airway-W- Philip Churchill [51] Int. Cl. ..B03b1/04, COlb 35/00 58 FieldoiSearch ..209/s,3,9;21o/37,1o; ABSTRACT 23/110 P; 106/72 288 A process for beneficiating a dispersed kaolin clay mass by removing from a dispersed clay mass the contaminant which [56] References Cited causes the formation of a thin, dull layer of yellow or UNITED STATES PATENTS brownish-yellow material, commonly referred to as scum," on surfaces of dried material containing the dispersed clay. 1,333,701 1 1/1932 Slmcoe -23/110 x The process comprises raising the pH of an aqueous dispersion 2/1939 Lyons "23/1 10 of the clay to at least 12 and at that pH separating the insolu- 2,509,680 I0/195l Leek x ble clay from the solubilized discoloring contaminant. 3,371,983 3/1968 Maynard... ..23/1 10 3,432,030 3/1969 Olivier ..209/5 6 Claims, No Drawings SEPARATION OF DISCOLORANTS FROM CLAY BACKGROUND OF THE INVENTION 1. Field of Invention This invention is concerned with a process for minimizing the amount of discoloring material in dispersed kaolin clay which appears on the dried surfaces of material coated with the dispersed clay or with dispersed clay-containing material. The invention is particularly important for improving coatings containing dispersed clay such as paper coatings and paints. The effect of this discoloring material can be easily and dramatically demonstrated by preparing a coating color containing a dispersed clay and an adhesive (e.g., a latex adhesive as Dow latex 620), applying the coating to both sides of a glass plate and merely visually observing the dried surfaces.

2. Description of the prior art When dispersed clays are dried, or when dispersed paper coatings or paints containing clays are dried, a thin dull layer of yellow or brownish-yellow material commonly referred to as scum" may appear on the surface of the dried material. This scum detracts from the brightness and white appearance of the dried mineral and has, as a result, posed a serious problem especially to the clay industry. Because of the current high specifications for clay demanded by clay users in the paper and filler industries, clays which in the past have normally fulfilled their needs are now being rejected because of the presence of this scum.

Several processes have been disclosed in the patent literature for improving the brightness of clays. U. 5. Pat. No. 3,353,668 discloses a process which involves incorporating a water soluble inorganic compound, which contains in its molecule readily available oxygen, into an aqueous clay slip at a pH below 5 and agitating the slip until the colored impurities in the clay are oxidized. US. Pat. No. 3,371,988 discloses removing titanium impurities from a clay by subjecting a slurry of clay to an overdose of peptizing agent beyond the amount necessary to achieve minimum viscosity.

US. Pat. No. 2,251,256 discloses an improvement in color of an aqueous clay slip by treating an aqueous suspension of a clay with a water-soluble bleaching-metal-salt of hydrosulphurous acid, said metal being replaceable to produce precipitated solids, followed by a thickening of the suspension through addition of an alkaline reagent in an amount sufficient to increase the pH of the suspension to about 10. Alternatively, a clay suspension may be subjected to successive treatments with the water-soluble bleaching-metal-salt of hydrosulphurous acid and with excess alkali up to the thickening point of the suspension. It is indicated in US. Pat. No. 2,251,256 that the alkaline reagent displaces the metallic radical of the hydrosulphurous acid compound to produce an insoluble material.

The above processes have inherent disadvantages since they either require the addition of external reagents, under carefully controlled conditions, which can be time consuming and expensive, or because they simply are ineffective for eliminating the scum.

OBJECTS OF THE INVENTION It is an object of this invention to produce a dispersed clay mass and a dispersed clay-containing coating and filler whiter than conventionally processed clay material, as shown by a mere visual comparison.

It is yet another object of the invention to remove from dispersed clay that material which produces a scum on surfaces of the dried dispersed clay-containing material, and particularly on dried coatings containing dispersed clay, and to produce a material generally with improved GE brightness.

It is a further object of the invention to produce a whiter dispersed kaolin clay by a simple, efficient and economical process.

It is yet another object of the invention to allow the production of dispersed clay products at a lower cost because of the need for smaller quantities of the standard clay bleaching agents to meet stated specifications,

SUMMARY OF THE INVENTION We have discovered that scumming is caused by discoloring organic compounds which are dispersed in naturally occurring clay. Apparently those discoloring compounds migrate to the surface as the dispersed clay or any material containing the dispersed clay is dried. We have further discovered that contrary to the teachings of the prior art, those discoloring impurities can be minimized by merely raising the pH of an aqueous dispersion of clay to at least a pH of 12 and separating the insoluble mass of clay at that pH from the solubilized discoloring contaminant. The clay, if desired, may then be subjected to conventional clay processing, e.g. bleaching with zinc or sodium hydrosulfites at a pH of about 4.

We have discovered that the discoloring impurities are absorbed at the clay surface in solutions of low pH, and so under the customary operating conditions where the clay is dewatered at an acid or near neutral pH, the discoloring impurities remain with the separated solid clay. In alkaline dispersions (pH at least 12), however, the discoloring impurities are brought into solution. The heart of the invention is the dewatering of the clay at a pH of at least 12 so that the discoloring impurities are removed with the water.

In our process clay is dispersed using conventional dispersing agents such as sodium silicate, tetrasodium pyrophosphate, sodium hexametaphosphate, sodium tripolyphosphate, sodium potassium butyl phosphate or other conventional dispersing agents to obtain maximum dispersion. The dispersed clay slurry is then subjected to a conventional procedure for removal of sand, mica and other coarse impurities. The pH of a dispersed clay slurry will usually be in the 68 range, depending on the dispersant used. For commercial applications of clay, dispersed clay dispersions at a pH of 6-8 are customarily used. Conventional classification (i.e. sedimentation, centrifugal classification, etc.) of the clay can also be accomplished at this point, if desired. The dispersed clay slurry (before or after cleaning and classification) is then treated with a water soluble alkaline material to raise the pH of the slurry to 12, thereby solubilizing the undesirable organic contaminant. The insoluble mass of clay is then separated from v the solubilized scum-forming material at this high pH. This can be done by subjecting the aqueous clay slurry to any conventional dewatering technique, e.g. a centrifugal field in which the dispersed clay is concentrated in the underflow and the solubilized organic contaminant removed with the water in the overflow, or simply a filtration which allows separation of the solubilized material from the slurry of insoluble mineral. Alternatively, the solubilized organic contaminant may be removed by adsorption of the discoloring component on an anionic ion exchange column. The above procedures can be repeated several times as desired to achieve a complete removal of the solubilized organic material from the clay.

We also have found that by adding an alkali to neutralize the acid pH and to bring the pH of the slip to at least 12 at which pH the discoloring impurities are rendered soluble, the concentration of electrolyte in the slip is high enough to cause some flocculation of the clay, thus making its separation by centrifuge or by filtration much more easy to effect.

In the process of the invention any basic material capable of raising the pH of the clay slurry to at least 12 can be used. 11- lustrative of such materials are alkali metal hydroxides such as potassium hydroxide and sodium hydroxide and alkaline earth metal hydroxides such as calcium hydroxide.

In comparison with processes described in the patent literature for improving color of clay, our invention is extremely simple. It requires no addition of external oxidizing agent or peptizing agent. It is not dependent on the presence of a water-soluble bleaching-metal-salt of hydrosulphurous acid, our addition of alkali to a pH of at least 12 being carried out without requiring the presence of an external bleaching agent.

Having thus described the invention andin order to gain a better understanding of its details, the following examples are given, not by way of limitation, but merely of illustration, in order to show the invention in its more specific aspects. All percentages are by weight unless otherwise specified.

EXAMPLE I Crude clay was slurried in water to which a dispersing agent (sodium silicate) had been added and the resulting dispersed slurry was degritted (sand, mica, etc. removed) and then passed through a centrifuge and a clay fraction was obtained having the particle size distribution shown in Table l. A portion of this degritted, centrifuged slurry was retained in its original state to serve as a control (pH 6.2 at 16 percent solids). The slurry pH of another portion was increased from 6.2 to 12.0 with sodium hydroxide and passed through a Merco Model 9 Centrifuge (Run 2). Under flow solids were 30-31 percent and overflow solids were 0.5-2.0 percent.

The underflow from Run 2 was reduced to about 15.5% solids by the addition of water. The diluted slurry was again passed through the Merco Centrifuge, with wash water being added in the Merco Centrifuge at a rate of 2 gal. water per minute as the slurry was being fed to the centrifuge at a rate of 1.17 gal./min. (Run 3).

The pH of each underflow sample (Runs 2 and 3) was then adjusted to about 4 with sulfuric acid. The samples from each of those underflows and the control were then treated with the amount of bleaching agent (zinc hydrosulfite) shown in Table I. Portions of the samples were then dispersed to maximum dispersion (as determined by adding the amount of dispersant required to give minimum viscosity) and dried in the dispersed state; to determine the amount of scumming that occurred.

In yet another determination latex coating colors were prepared by blending 286 g. of a 70 percent solids slurry of the control clay sample (bleached with 5 lbs. zinc hydrosulfite per ton of clay) with 40 ml of 50 percent solids Dow 620 latex and a similar coating color was prepared using the product from Run 2 (bleached with 5 lbs. zinc hydrosulfite per ton of clay). The coating color was used to coat a glass plate (both sides coated) using a 3 mil Bird film applicator. The coatings were then dried.

The pH before centrifugation, the particle size distribution and GE brightness of the control and clay from Run Nos. 2 and 3 (unbleached and bleached) are set out in Table No. I,

infra.

Table I No. 2 No. 3 Run No. Control Under-25 Under flow flow pH 6.2 12.0 12.0 GB. Brightness at various bleach levels lbs. zinc (hydrosulfite/dry ton clay) 3 87.4 88.1 4 87.4 88.7 5 87.7 88.1 6 87.8 88.3 Particle Size Distribution (E.S.D.) Percent below indicated size 5 micron 97.0% 98.0% 98.5% 2 micron 83.0% 82.0% 83.0% 1 micron 67.0% 64.0% 66.0% 6.5. Brightness oflatex coating 78.5 79.3 Chemical Analysis Fe 0: 0.272 0.356 TiO 1.29 1.27

By mere visual observation the surfaces of the control (unbleached, and bleached) and the latex coating color prepared from the control had a very bad layer of discoloring scum. In comparison, the surfaces of both the unbleached and bleached samples 01 Run Nos. 2 and 3 and latex coating color from Run No. 2 sample had showed a substantial improvement. 0f significance are the showings in Table I that the improved sur face characteristics of the samples are not attributed to particle size deviation or decrease in T102 or Fe O content.

To establish the necessity of raising the pH of the clay slip before centrifuging in order to obtain scum reduction, a sample of a dispersed clay fraction (pl-1 6.2) similar to the control, supra, was passed through the Merco under conditions similar to those mentioned above. The overflow (ca. 2 percent solids) was not particularly yellow. The underflow (ca. 30 percent solids) was then acidified, bleached, dispersed and dried as outlined previously. No improvement in scum reduction was observed in the centrifuge sample.

EXAMPLE I] A dispersed clay slip at 14.9 percent solids, similar to that used in Example I, was treated with sodium hydroxide (NaOI-I) until the pH of the slip was 12.0. The slurry was then passed through the Merco centrifuge and a sample of the underflow was obtained (Run 1). The remainder of the underflow was diluted to 12.8 percent solids, again passed through the centrifuge and a sample of the underflow retained (Run 2). The rest of the Run 2 underflow was diluted to 15.7 percent solids and passed once more through the centrifuge and an underflow obtained (Run 3).

Samples of the starting feed material and the underflow from the various runs were adjusted to pH 4.0 t 0.2, using sulfuric acid, and were then treated with sodium hydrosulfite (3 lb./ton dry clay equivalent). The samples were then filtered, reslurried in water at about 25 percent solids, refiltered and dried. Coatings were prepared (latex drawdown method as in Example 1 from the clays. GE brightness and particle size distribution, are set out in Table II infra. In addition, viscosity data of the samples were determined and standard casein coatings were applied to paper to ascertain whether any adverse effect on other clay properties had occurred. The resultant data, including G.E. brightness and gloss values of the casein coatings, are also set out in Table 11.

TABLE II Material Control Run 1 Run 2 Run 3 Percent Solids of Feed 14.9 14.9 12.8 15.7 pH 6.2 12.0 12.0 12.0 Percent Solids of Underflow 30.6 29.5 32.1 pH of Underflow 11.5 11.6 Percent Solids of Overflow 1.0 1.23 Lbs. sodium hydrosulfite/ton dry clay 3 3 3 3 G.E. Brightness of Clay 86.7 87.1 87.9 88.0 Particle Size Distribution (E.S.D.) Percent Finer than Size shown 5 microns 97 97 97 98 2 microns 81 83 83 l microns 63 65 60 64 Stormer Viscosity (see/100 rev.) at minimum viscosity and at solids shown 74% 24.4 20.2 21.8 25.2 72% 15.4 14.2 14.2 14.0 70% 11.8 11.2 11.0 12.0 65% 8.4 7.8 8.4 8.5 Brookfield Viscosity, (eentipoise at 70% solids and No. 2 Spindle) 10 RPM 156 153 140 160 20 RPM 124 126 124 50 RPM 86 100 114 100 RPM 108 I21 GE. Brightness of Latex Drawdown 78.5 78.8 79.4 80.2 Coated Paper Properties G.E. (uncalendered) 79.2 79.6 80.2 80.5 Gloss (calendered sheet) 58.5 60.0 67.5 68.5

Visual observation showed the surfaces of samples from Run Nos. 1, 2 and 3 clays, and casein and latex coatings from those samples substantially free of scum in comparison with surfaces of the control sample and latex and casein coatings from the control sample.

EXAMPLE 111 A clay dispersion was prepared as outlined in Example 1, except that a product of finer particle size distribution was obtained from the classification step. A portion of the dispersion at 15.5 percent solids was adjusted to pH 12.5, using NaOH, and passed almost immediately through the Merco centrifuge (Run 1). Another portion of the slurry was adjusted to pH 12.5, using NaOH, and then agitated for 24 hours before being passed through the centrifuge (Run 2). A portion of the slip TABLE 111 Material Control Run 1 Run 2 Run 3 pH 6.2 12.5 12.5 12.85 G.E. Brightness at various bleach levels lbs. zinc hydrosulfite/dry ton clay 3 88.1 88.4 4 88.2 88.7 5 88.3 88.4 6 87.0 88.4 89.0 89.3 Particle Size Distribution (E.S.D.) Percent below indicated size 5 microns 99 99 99 99 2 microns 96 95 95 95 1 microns 81 79 79 80 Tristimulus Values Visual Efficiency 92.4 92.8 Color (lower number indicates whiter material) 3.6 3.3 Dominant Wavelength (my) 576 576 Latex drawdown 79.3 81.4 Coated Paper Properties G.E. Brightness (uncalendered) 80.2 80.6 Gloss (calendered) 75 73 The control bleached and unbleached and by visual observation had a heavy layer of scum. In comparison Run Nos. 1, 2 and 3 bleached and unbleached were substantially free of scum.

EXAMPLE IV A sample of clay similar to that used in Example 111 was prepared at a 16 percent solids dispersed slurry. It had a pH of 6.3. A portion was adjusted to a pH of 12.0 (Run No. 1) using sodium hydroxide (12.8 lbs. of sodium hydroxide per ton of dry clay). A second portion of the slurry with pH of 6.3 was adjusted to 12.5 (Run No. 2) (31.8 lbs. of sodium hydroxide per ton of dry clay). A third portion of that slurry was adjusted to a pH of 13.0 (Run No. 3) (1027 lbs. of sodium hydroxide per ton of dry clay).

Samples of the above portions with pH of 12.0 (Run No. 1 12.5 (Run No. 2) and 13.0 (Run No. 3) were then centrifuged, using a Tolhurst centrifuge (12 inch center slung, with imperforate basket) at approximately 750 G. In each of the runs, the centrifuge was initially charged and, after timed intervals, the overflow was withdrawn by means of a skimmer tube and feed added in the same amount as overflow withdrawn. Properties of the samples obtained are shown in Table IV infra.

TABLE IV Material Control Run 1 Run 2 Run 3 pH 6.3 12.0 12.5 13.0 Solids, 16%

Overflow 1.0 0.8 0.2 Underflow 72.6 73.0 625 Chemical Analysis (Underflow) TiO 1.41 1.30 1.45 1.37 Fe O 0.332 0.312 0.318 0.326 Particle Size (M is micron) (Underflow) Below 5.0 M. 99.0 98.5 99.0 99.0 Below 2.0 M. 96.0 94.5 95.5 95.0 Below 1.0 M. 81.0 75.0 81.0 81.0 %Below0.5 M. 53.0 47.5 52.5 49.0

Samples of the underflow materials from centrifugation, as well as the control, were flocculated to pH 4.0 i 0.2, using sulfuric acid and were then filtered. A portion of each filter cake was dried so that an unbleached G.E. brightness value could be obtained. Also, other portions of each of the filter cakes were dispersed to minimum viscosity, then dried in dispersed form so that the presence or absence of scum could be observed. The G.E. brightness values are given infra.

Material Control Run 1 Run 2 Run 3 G. E. Brightness (Unbleached) 85.9 87.0 86.4 86.0

Visual observation showed the control to have a heavy layer of scum. In comparison, the runs from pH of 12.0, 12.5 and 13.0 were substantially free of scum. v

Various amounts of bleach (zincv hydrosulfite) were added (at a slurry pH value of 4) to each of the products and to the control. The samples were then filtered and dried. Various tests were performed on the samples and the results are given in Table V.

Visual observation showed the control bleached at 6 lb./ton zinc hydrosulfite level or unbleached to exhibit a heavy layer of scum. In contrast, runs from pH of I20, 15.5 and 13.0 unbleached or bleached as the control were substantially free of scum. The same contrasts were observed in the coatings.

EXAMPLE V A somewhat coarser clay fraction (one similar to that used in Example I) was subjected to a series of tests as reported for in Example IV. Results are given in Table VI which specify the amount of NaOH required to reach a pH of 12.0 and 12.5, and in Table VII which lists the properties of a control sample and the samples produced through use of the process of the invention.

TABLE VI Lbs. pH NaOH/ton clay As rec'd 6.8

TABLE VII pH pH Material Control l2.0 Run 12.5 Run 40 G.E. Brightness at Various Bleach Levels (Lbs. sodium 2 86.0 88.0 87.6 hydrosulfite per 3 85.9 87.7 87.2 dry ton of clay) 4 85.7 88.1 87.9 Particle Size Distribution (E.S.D) Percent below indicated size 5 microns 97 9s 97 2 microns 82 80 81 l microns 65 63 65 Chemical Analysis Fe O 0.328 0.0336 0.346 TiO l.30 1.26 1.36

EXAMPLE VI Some 150 tons of a fine clay fraction similar to that disclosed in Example III was obtained by preparing a dispersed slurry of crude clay, degritting the slurry, then passing the slurry through a centrifuge to obtain a clay fraction with a particle size distribution (E.S.D.) of 99 percent below 5 micron, 95 percent below two micron and 78 percent below 1 micron. The slurry (at 14 percent solids) was treated with enough sodium hydroxide (ca. 20 lbs. NaOH/dry ton clay) to raise the pH of the mixtureto 12.3. The mixture was then passed through De Laval. centrifuges. Overflow solids were ca. 2-4 percent. The underflow at ca. 30 percent solids, which contained most of the clay, was collected and acidified, using sulfuric acid, to a pH value of 3.8. The clay was bleached by the addition of a solution of zinc hydrosulfite (4 lbs. dry zinc hydrosulfite/ton dry clay) and then filtered on large rotary vacuum filters, with wash water being applied to the filter cake during filtration. The surface of the dried dispersed product had substantially no scum in contrast to a control sample, which had a heavy, dark scum. The G.E. brightness of the product from the slurry having a pH of 12.3 was 87.8, whereas control samples (starting feed bleached) had brightness values ranging from 86.5 to 87.0.

EXAMPLE VII A finer clay fraction was dispersed in water, using tetrasodium pyrophosphate as dispersant and was delaminated according to the process given in US. Pat. No. 3, l 7 l ,7 l 8, to obtain a delaminated clay product as described in Example 10 of the referenced patent. The slurry was then treated with a solution of sodium hydroxide to raise its pH to 12.5. The basic mixture was centrifuged, using a Tolhurst basket centrifuge (12 inch center slung) with imperforate basket. An overflow (at 0.5 percent solids) was removed through use of a skimmer tube. The underflow, containing most of the clay, was diluted and enough sulfuric acid was added to lower the pH to 4. The sample thereafter was bleached in the conventional manner, using zinc hydrosulfite, filtered, reslurried in water (a washing step), re-filtered and dried. Physical properties of the treated product and of a control are given in Table VIII.

A fine clay fraction similar to that used in Example III was dispersed in water, using sodium silicate as dispersant, and was then treated with enough potassium hydroxide to raise the pH of the slurry to 12.0. A portion of the sample was filtered and the filtrate was found to have a marked yellow color. The filtrate was passed through an ion exchange column prepared using Dowex Anion exchange resin which had previously been treated with a solution of potassium hydroxide. The yellow color was completely adsorbed on the resin while the liquid which was recovered from the column was colorless. A portion of the clay slurry at pH 12 was then passed through the column, the insoluble clay going through the column. In this case, the water of-the clay slurry was colorless, indicating that the soluble yellow organic contaminant had been adsorbed by the anion exchange resin and thereby removed from the clay slurry.

Infrared study of a concentrate of the yellow organic impurity indicated the presence of aliphatic and aromatic CH groups, a very broad adsorption band in the O-H and NH region, adsorption in the C O, C C and/or C-NH: region and an adsorption band in the 1490 cm region.

EXAMPLE 1X Table IX.

TABLE 1X Material Control Treated Product pH 6.2 12 G.E. Brightness of clay at 87.8 88.7 bleach level of 6 lbs. zinc hydrosulfite per dry ton clay Particle Size Distribution (E.S.D.) Percent below indicated size microns 99 99 2 microns 95 96 I microns 79 85 (LE. Brightness of Latex Coating 78.0 80.7 Coated Paper (as Example 111) Properties G.E. Brightness (uncalendered) 80.2 81.0 Gloss (calendered) 75.0 75.0 Chemical Analyses 7o Fe 0 0.298 0.266 TiO 1.39 1.39

Visual observation showed that the control (unbleached and bleached) and latex and paper coatings from the control to have a substantial layer of scum, in contrast to the absence of the scum in the treated product (unbleached and bleached) and coatings from the treated product.

EXAMPLE X Run a:

Six gallons of a clay fraction similar to the one used in Example lll (control) at 17 percent solids were treated with sodium hydroxide to a pH of 12.0. The treated feed was agitated and heated for one hour at 180 F by using a steam heated copper coil immersed in the slip. After cooling, the slip pH was 1 1.8 and the solids were 21 percent. The pH was readjusted to 12.0.

The treated slip was centrifuged (batch-wise) in the T01- hurst Basket Centrifuge (Tolhurst Centrifuge, 12 inch Center Slung, with an lmperforate Basket) SN ZT-37l75-A. The centrifuge RPM was set on 2100 (750 G's). After timed intervals, overflow was withdrawn by means of a skimmer tube and treated feed added back in the same amount as-overflow withdrawn.

Run b:

Three gallons of the clay used in run a (control) at 18.7 percent solids were treated with sodium hydroxide to a pH of 12.0. The treated feed was conditioned in a Kady Mill. Nominal retention time in the Kady was minutes and the temperature during conditioning was 6065 C. After conditioning, the pH was 1 1.8. It was readjusted to pH 12.0.

The conditioned slip was centrifuged in the same manner as described for run a above.

Tests Before Filtration or bleaching Run a Run b (Heated) (Not Heated) Solids,

Overflow l .2 1.5 Underflow 73.0 74.4

Dried dispersed samples from runs a and b were substantially free of scum. In comparison the control (starting feed) had a heavy layer of scum.

Run 0:

Bleach series were made on each of the underflow products from runs a and b, supra. Samples were diluted to 23 to 28 percent solids, flocculated with sulfuric acid to 4.0 r 0.2 and bleached ZnS O as follows:

Pounds of Zinc G.E. Brightness Hydrosulfite Per Run a Run b Ton of Clay (Heated) (Unheated) The sample underflow (Run a) was divided into two parts and processed as follows:

The underflow product was diluted to 25.5 percent solids, flocculated to pH 4.0 t 0.2, bleached at 4 lbs. per ton (ZnS-,O,, filtered, dried and pulverized (Run a-l The second portion of underflow was diluted to 25 percent solids, flocculated and filtered. The filter cake was reslurried to 25 percent solids, pH adjusted to 4.0 i 0.2, bleached at 4 lbs. per ton (ZnS OQ, filtered, dried and pulverized (Run a-2).

The underflow sample (Run 12) was diluted to 28 percent solids, flocculated, bleached at 4 lbs. per ton (ZnS O,), filtered, dried and pulverized (Run b-l Chemical analysis and paper coating tests and latex drawdowns were made. Following data were obtained:

Chemical Analysis Control Run a-l Run a-2 Run b-l (Starting Feed) S0 0.07 0.17 0.30 0.25 K 0, 0.02 0.014 0.02 0.003 CaO, 0.015 0.01 0.015 0.018 TiO 1.25 1.24 1.23 1.39 FeO 3% 0.365 0.358 0.33 0.298 ZnO, 0.04 1.035 0.048 0.061

Control Run al Run a-2 Run b-l (Starting Feed) G.E. brightness 88.4 88.3 88.8 88.0 Particle Size: (M stands for micron): Below 2.0 M. 96.5 96.0 96.0 94.5 Below 1.0 M. 86.0 86.0 78.0 78.5 Below 0.5 M. 58.0 57.0 50.0 52.0

Coated Paper Properties Control Run a-l Run a-2 Run b-l (Starting Feed) Gloss: Cal. 74.7 74.3 68.5 75.2 6.15.: Uncal. 81.0 81.0 80.4 80.2 Cal. 77.2 77.9 77.3 77.0 Latex Drawndown: 80.7 80.9 80.0 78.0

The dried dispersed bleached samples from the control and Tests Before Filtration or Bleaching latex coating from the control had a dark scum layer. In con- 4 trast, samples processed according to the present invention Sample Base and latex coatings from such samples were substantially free ofscum. Control 185 (pH 5.5) EXAMPLE X1 Oflow U'fiow a KOH 1.0 70.3

l a. A 3.5 gallon sample of unbleached l aolin clay (sodium 2 $23.; 3 Z23 silicate dispersed) feed (18.5 percent solids at pH of 5.5; the 10 d Butylcontrol) was treated with potassium hydroxide (24 grams) to a afnim L2 pH of 12.0. The treated sample was then centrifuged in a Tole l 2 73 0 hurst Basket Centrifuge, 12 inch Center Slung, with Imperf 0 2 1:0 7:20 forate Basket. The centrifuge RPM was set on 2100 (750 6'5). g N l-l f (bleached A or initially charging the centrifuge, overflow was 15 clay) 0.6 7L2 withdrawn by means of a skimmer tube and treated feed added back in the same amount as overflow withdrawn.

Sample heavily flocculated. The overflow was clear.

CHEMICAL ANALYSIS U'FLOW (UNBLEACHED) Percent 0. 069 0. 16 O. 023 O. 01 l. 38 ll. 371 (l. (118 0. 032 0. 14 O. 293 0. 014 l. 36 1]. 29% (l. 001! (l. ()5 (l. 075 0. 02 0. 58 l. 37 (l. 392 i). 01-! U. 06 U. 183 0. 023 l). 021 1. 31! (l. 365 I). 015 U. 068 U. ()5 O. 02 l). 00) l. 34 U. 365 0. U15 0. U5 0. 265 0. 023 0. 022 l. 32 U. 356 U. 015 0. 044 O. ()4 U. 021 U. 0118 l. 32 U. 3". U. U15 U. 038 O. 186 (1.022 0. 01 1. 3(1 0. 3'21 0. 03'.

l Bleached clay.

b. A 10 liter sample of unbleached kaolin clay (sodiumsil V m Tes Unbleached icate dispersed) feed 19.2 percent solids at pH of about 5.5) was treated with calcium hydroxide grams calcium hydroxide slurried in 1 liter of water) to a pH of 12.0 and 3 5 Sample Base Scum stirred for l hour. The treated sample was then centrifuged as described above. c 1 B d d k i OlIUO a a! c. A 10 liter sample of unbleached kaolin clay (sodium Slla KOH Light Scum icate dispersed) feed 19.2 percent solids at pH of about 5.5) b m-1 v lime, if any scum was treated with ammonium hydroxide (1.5 liters concen- C NFLOH Light s i d Butylarnine ,Slight scum trated ammonium hydroxide) to a pH of U5. The treated e NaOPHCaOHh slightscum sample was then centrifuged as described above. f NaOH Sigh! Scum d. A 10 liter sample of unbleached kaolin clay (sodium silg NaOH (bleached Slight scum icate dispersed) feed (19.2 percent solids at pH of about 5.5) I clay) was treated with N-butylamine (300 milliliters) to a pH of 11.9. The treated sample was then centrifuged as described above. h. A bleach series was conducted on each of the underflow e- A 1 li r a pl Of unbleached kaolin l y (Sodium silproducts and control. The products were diluted to 24 to 26 icate dispersed) feed (18.5 percent solids at pH of about 5.5) percent solids with deionized water and bleached at three was treated with sodium hydroxide (12.7 grams) and calcium levels.

BLEACH SERIES, GE

Sample a b c (l e l g Butyl- NnOH+ Base Control KOH C21.(OH)2 NH4OII amine Cn(OH); NaOIl N30 11 1 0 Unbleached 84. G 85. 1) 84. 7 85. 8 81. 8 85. l 85. 3 86. 3 lbs. zinc hydrosullite per ton 85. 1 87. 5 86. 6 86. 9 86. 5 86.8 85. .2 87. 5 4 lbs. Zinc hydrosullltc per ton 86. l) 88. 0 86.8 87. 0 8G. 8 87. l 87. 3 87. 6 5 lbs. zinc hydrosulfite per ton 8G. 9 87.7 86. 7 86. O 87. 5 86. 9 87. 4 87. 5

6 lbs. zinc hydmsulfite per ton 86. 8 .1

1 Bleached clay.

hydroxide (0.1 grain) to a pH of 12.0, then centrifuged as Each of the products and control was flocculated with suldescribed above. furic acid, with the exception of above sample (1)), to a pH of f. A 10 liter sample of unbleached kaolin clay (sodium sil- 4- i -2. The pH of Sample was 50 adjusted with icate dispersed) feed (18.5 percent solids at pH of about 5.5) hy i acid (Calcium Chloride being much more Water was treated with sodium hydroxide (12.2 grams] to a H f soluble than calcium sulfate). Each of the samples were 12.0, then centrifuged as described above. bleached at 4 lbs. zinc hydrosulfite per ton of clay, except the g. A 2000 gram sample of kaolin clay, bleached at 4 lbs. per n r l mpl n pl h se were bleached at 5 ton (ZnS O was slurried at 18 percent olid a d di d zinc hydrosulfite per ton of clay. After bleaching, the samples with 0.2 percent sodium silicate H 7 O), Th sample was were filtered, dried and pulverized. These samples were subtreated with sodium hydroxide (14.6 grams) to a pH of 12,0, mitted for chemical analyses and paper tests, including latexthen centrifuged as described above, glass drawdowns. Scum tests were also made on the finished product.

Scum Tests (Unbleached) Control Scummed, bad

Light scum Light scum Light scum Light scum QUO'N e. A bleach series was conducted on the underflow products and control. The samples were diluted to 26 percent solids with deionized water and bleached at three levels, with the exception of the control, which was bleached at four levels.

Bleach Series, G.E.

Sample Description Control a b c Unbleached 85.1 3 lb. zinc 85.2 hydrosulfite per ton 4 lb. zinc hydrosulfite per ton 5 lb. zinc hydrosulfite per ton 6 lb. zinc hydrosulfite per ton BLEACHED PRODUCTS AND CONTROL Sample Control I! c cl pH 3.2 3.6 3.4 3.8 3.2 G.E. Brightness 87.1 87.2 87.5 88.2 87.3 Particle Size Below 50 M. 98.0 98.5 99.0 98.0 99.0 Below 2.0 M. 94.5 95.0 92.5 94.0 95.0 Below l.0 M. 8L0 80.0 78.0 78.0 80.0 Below 05 M. 54.0 54.5 52.0 53.0 52.0

Paper Coating Data Gloss: Cal. 742 71.3 71.7 72.5 76.3 (1E. Brightness Uncal. 79.8 81.3 81.2 31.5 81.2 Cal. 76.7 78.1 78.2 78.0 76.9

(3.13. Brightness of latex drawdown 78.1 78.7 80.1 80.0 78.8 Scum scummed, light light light light not as scum scum scum scum dark as unbleach- Chemical Analysis (Bleached) Sample Description Control a b c d P O 0.085 0.04 0.056 0.04 0.052 SO;,,% 0.233 0.106 0.26 0.283 0.11 K O,% 0.019 0.02 0.021 0.02 0.022 CaO,% 0.01 0.008 0.01 0.02 TiO;,% 4 1.44 1.35 1.35 1.30 1.31 Fe O 0.334 0.33 0.30 0.307 0.32 ZnO,% 0.059 0.057 0.032 0.045 0.042

For brevity, various features have been illustrated and described in connection with a limited number of illustrative embodiments. It is intended that these features be used also in combination with features shown in other embodiments without departing from the spirit of the invention.

The terms and expressions that have been employed in the specification are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, since it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

l. A process for removing these discoloring contaminants associated with a kaolin clay which produce a thin dull layer of a yellow or brownish-yellow scum on the surface of dry degritted kaolin clay or coatings prepared from said degritted clay, which comprises:

1. dispersing said degritted kaolin clay containing said scum-forming contaminants in an aqueous medium having a pH ofat least 12;

2. maintaining said clay in contact with said aqueous medium at a pH of at least l2 until said contaminants become solubilized in said medium and are thereby liberated from the clay, and;

. then separating by a process such as filtration, centrifugation or ion exchange the insoluble kaolin clay from the aqueous medium in which the contaminants are solubilized while maintaining the aqueous medium at a pH of at least 12, to recover a clay product characterized by a markedly decreased tendency to form a surface scum layer when said clay is dried,

said process further characterized in that it produces no substantial decrease in the TiO- and Fe O content of the clay.

2. Process according to claim 1 in which the separation is caused by a centrifugal field in which the dispersed clay is concentrated in the underflow and the solubilized discoloring contaminant in the overflow.

3. Process according to claim 1 in which the separation is caused by filtration of the insoluble clay.

4. Process according to claim 1 in which the separation is caused by an adsorption of the solubilized discoloring contaminant onto an anionic ion exchange resin.

5. A process for removing those discoloring contaminants associated with a kaolin clay which produce a thin dull layer of a yellow or a brownish-yellow scum on the surface of dry degritted kaolin clay or coatings prepared from said degritted clay which comprises:

1. dispersing said degritted kaolin clay containing said scum-forming contaminants in an aqueous medium having a pH of at least l2, said medium comprising a solution selected from the group consisting of aqueous alkali metal hydroxide solutions and aqueous alkaline earth metal hydroxide solutions; maintaining said clay in contact with said aqueous medium at a pH of at least 12 until said contaminants become solubilized in said medium and are thereby liberated from the clay, and,

3. then separating by a process such as filtration, centrifugation or ion exchange the insoluble kaolin clay from the aqueous medium in which the contaminants are solubilized while maintaining the aqueous medium at a pH of at least 12 to recover a clay product characterized by a markedly decreased tendency to form a surface scum layer when said clay is dried,

said process further characterized in that it produces no substantial decrease in the TiO and Fe o content of the clay.

6. The process of claim 5 wherein said alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide and said alkaline earth hydroxide is calcium hydroxide.

Pawn t NO. $659,708 Dated y 1972 Inventor(s) a H. Morris et al.

It is certiiied that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line +7, after Under-" delete '25";

Col. 6, line 57, delete "pH 12.0" over "Control";

Col. 6, line 5?, insert pH 12.0 over "Run";

Col. 7, line 13, "15.5" should be 12.5 ---5 C01. 9, lin 73, delete "bleaching" over Run b" and insert bleaching after "or";

Col. 10, line 5 "1.035" should be 0.035

Col. 10, line 73, "Drawndown" should be Drawdown Col. 12, line 7, "70.3" should be +9.0

Col. 13, line 35, FeO should be Fe O (in the percent table) 001. 14,-line 67, "0007" should be 0.007 (in the sodium potassium table) Col. 15, line #3, replace "b" by a ----5 (in the Bleached Products table) Col. 15, line L3, insert letter b over numerals "3.4" (in the Bleached Products table) in line 46;

F OHM IOJUSO (10-69} UMTED STATES PATENT GFFICE CERTEFICATE 0F CQRRECHON 3, 59,70 Dated Mav 2. 1972 H. H. Morris et a1.

Patent No.

Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 15, line 7A, "ALMA" should be 1. m

Col. 16, line l t, these should be those Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. Process according to claim 1 in which the separation is caused by a centrifugal field in which the dispersed clay is concentrated in the underflow and the solubilized discoloring contaminant in the overflow.
 2. maintaining said clay in contact with said aqueous medium at a pH of at least 12 until said contaminants become solubilized in said medium and are thereby liberated from the clay, and;
 2. maintaining said clay in contact with said aqueous medium at a pH of at least 12 until said contaminants become solubilized in said medium and are thereby liberated from the clay, and,
 3. then separating by a process such as filtration, centrifugation or ion exchange the insoluble kaolin clay from the aqueous medium in which the contaminants are solubilized while maintaining the aqueous medium at a pH of at least 12 to recover a clay product characterized by a markedly decreased tendency to form a surface scum layer when said clay is dried, said process further characterized in that it produces no substantial dEcrease in the TiO2 and Fe2O3 content of the clay.
 3. then separating by a process such as filtration, centrifugation or ion exchange the insoluble kaolin clay from the aqueous medium in which the contaminants are solubilized while maintaining the aqueous medium at a pH of at least 12, to recover a clay product characterized by a markedly decreased tendency to form a surface scum layer when said clay is dried, said process further characterized in that it produces no substantial decrease in the TiO2 and Fe2O3 content of the clay.
 3. Process according to claim 1 in which the separation is caused by filtration of the insoluble clay.
 4. Process according to claim 1 in which the separation is caused by an adsorption of the solubilized discoloring contaminant onto an anionic ion exchange resin.
 5. A process for removing those discoloring contaminants associated with a kaolin clay which produce a thin dull layer of a yellow or a brownish-yellow scum on the surface of dry degritted kaolin clay or coatings prepared from said degritted clay which comprises:
 6. The process of claim 5 wherein said alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide and said alkaline earth hydroxide is calcium hydroxide. 